Food and drink products and other perishable items are often packaged in tubular containers which are sealed at both ends. These tubular containers typically include at least one structural body ply and are formed by wrapping a continuous strip of body ply material around a mandrel of a desired shape to create a tubular structure. The body ply strip may be spirally wound around the mandrel or passed through a series of forming elements so as to be wrapped in a convolute shape around the mandrel. At the downstream end of the mandrel, the tube is cut into discrete lengths and is then fitted with end caps to form the container.
Tubular containers of this type typically include a liner ply on the inner surface of the paperboard body ply. The liner ply prevents liquids such as juice from leaking out of the container and also prevents liquids from entering the container and possibly contaminating the food product contained therein. Preferably, the liner ply is also resistant to the passage of gasses, so as to prevent odors of the food product in the container from escaping and to prevent atmospheric air from entering the container and spoiling the food product. Thus, the liner ply provides barrier properties and the body ply provides structural properties.
Conventional liner plies most often include aluminum foil which has good barrier properties and also has advantageous strength properties. In particular, the liner is wound onto the mandrel prior to the winding of the body ply and must be sufficiently strong and stiff to be independently wound on the mandrel without stretching or wrinkling. Because of the support provided by the foil layer of the liner, such liners are known as "supported" liners. One or more polymeric layers are normally adhered to the foil to further improve the barrier properties of the liner and it is sometimes the case that the foil layer is not necessary for barrier properties but is included in the liner only to provide support. Such foils are expensive and thus it is desired to provide an "unsupported" liner having the requisite barrier properties without the aluminum foil layer. However, because of the problems associated with winding an unsupported liner on the mandrel, such as stretching, creasing or other misshaping of the liner, it has not been commercially feasible with conventional winding apparatus and methods to manufacture a container having an unsupported liner ply.
Another problem associated with conventional liners is excessive friction between the liner ply and the mandrel as the tubular container is wound. If the amount of friction is too high, the mandrel can suffer from overheating or premature wear. Lubricants are used although the lubricants can be expensive and may present FDA concerns. Accordingly, it would be desirable to provide a tubular container which does not create excessive friction with the shaping mandrel. In addition, the aluminum foil layer typically includes a kraft paper backing for allowing the foil layer to be adhered to the paperboard body ply. Aqueous based adhesives (or "wet adhesives") are preferably being used to adhere the liner ply to the body ply because solvent based adhesives have become disadvantageous in light of various environmental concerns. However, it has heretofore been difficult to get the aqueous adhesives to stick to the smooth and impervious surface of the aluminum foil layer. Accordingly, a kraft paper backing has been preadhered to the foil layer so that the liner can be adhered to the paperboard body ply with wet adhesives. However, the kraft paper adds further cost and thickness to the liner.
The liner ply is sealed to itself along a helical seam which is typically slightly offset from the helical seam of the body ply. Wet adhesives have typically not been able to adhere directly to the foil layer as discussed above, and thus the liner ply seam is formed with an "anaconda" fold, wherein the overlying edge of the liner ply is folded back on itself and adhered to the underlying edge. The anaconda fold allows the polymeric layers on the surface of the foil layer to be heat sealed together. Alternatively, a hot melt adhesive can be used to seal the anaconda fold of the overlying edge of the liner ply to the underlying edge. An additional advantage of the anaconda fold is that the edge of the kraft paper is not exposed to the interior of the container and thus liquids in the container will not be absorbed by the kraft paper. An example of such a fold is illustrated in U.S. Pat. No. 5,084,284 to McDilda, et al.
Anaconda folds are undesirable, however, because of their increased thickness. The thickness of an anaconda fold seam is equal to three thicknesses of the liner ply and poses difficulties when attempting to hermetically seal the ends of the tubular container. Specifically, the ends of the tube are often rolled outwardly after being cut so as to form a rolled circular bead or flange on one or both ends of the tube and then end caps or covers are sealed to the bead with an adhesive sealant or compound. However, in the area where the thick anaconda fold seam forms a portion of the edge surface, the end surface of the bead or flange can be substantially non-planar thus forming hill-like and/or valley-like irregularities. Accordingly, an extra amount of adhesive sealant must be applied to the edge surface at least in the area of the anaconda fold seam to fill the discontinuities and hermetically seal the tubular container. The additional application of adhesive sealant is disadvantageous because of the extra sealant which must be used and the increased difficulty in removing the seal by the consumer due to the additional adhesive sealant.
Prior tubular containers having a liner without an anaconda fold seam include the container disclosed in U.S. Pat. No. 3,520,463 to Ahlemeyer. The container disclosed therein includes a liner ply of aluminum foil which is coated on one surface to inhibit chemical attack. The liner ply web is fed to a pair of combining rolls where its uncoated surface is forced into contact with an adhesively coated surface of a body ply web. Solvent based adhesives are disclosed and include animal glue, casein-latex emulsion, vinyl-copolymer emulsion, and sodium silicate. The composite web is then spirally wound into tubular form about a mandrel to create a continuous tube. The overlapping edges of the liner ply are secured together with a hot melt adhesive.
In addition, current commercial containers often have membrane-type lids or end closures heat sealed to a beaded end of the composite container wall. The beaded end is formed by turning out the end of the container liner to position the inner layer of the liner material on the outwardly curved beaded surface. As noted above, the liner material has conventionally been a paper-backed foil liner which is spirally wound inside the composite container, examples of which are disclosed in U.S. Pat. No. 3,892,351 to Johnson et al. and U.S. Pat. No. 4,280,653 to Elias. The foil liner and paper backing are not inexpensive and have an undesirable thickness.
Certain types of heat sealable coatings have been used in both the lidding and liners of conventional containers. For example, SURLYN.RTM. polymer, a product of Dupont, is a material known in the art and is commonly used as a heat seal coating. SURLYN.RTM. polymer is an ionically cross-linked polymer with limited flow characteristics when heated. Typically, the layer of the container and the layer of the membrane which contact each other are constructed of SURLYN.RTM. polymer, and may be coated with a wax. These two layers of SURLYN.RTM. polymer are heat sealed along the top surface of the container bead. The two SURLYN.RTM. polymer layers create an extremely strong bond layer that remains relatively uniform in thickness across the seal area. Due to the strong cross-linked bond created by SURLYN.RTM. polymer, however, opening the container can require a peel force which is too high for some consumers and usually results in tearing and exposure of the other layers of the container wall, such as the paperboard body wall, as is illustrated in U.S. Pat. No. 4,280,653 to Elias. This gives the top of the container a ragged, undesirable appearance.
Accordingly, it would be desirable to provide methods and apparatus for manufacturing a tubular container having an unsupported liner ply which does not include a foil layer. In addition, it would be highly desirable to provide such a container wherein the liner ply is securely adhered to the body ply with an aqueous adhesive. Such an aqueous adhesive would avoid the problems associated with solvent adhesives but should be capable of forming a sufficiently strong bond with the liner ply, a construction which has not yet been commercially feasible. In addition, it would be advantageous if the liner ply could be sealed without using an anaconda fold seam. A tubular container which could be wound without generating excessive friction with the shaping mandrel would also be very desirable.
It would also be advantageous to provide a sealed container and a method for sealing a container that combine ease of opening and an attractive appearance after opening with the seal strength and barrier properties required for protection of the products within the container.